Amid customary automotive manufacture, sheet metal workpiece layers are joined together to produce body panels such as those used in doors, hoods, trunk lids, and lift gates. Increasingly, in an effort to reduce vehicle body weight, these body panels are designed to include at least a pair of aluminum alloy sheet metal layers, rather than steel sheet. A series of individual resistance spot welds are usually made along an overlapping or other type of contacting interface to join the aluminum alloy sheet metal layers together, as one step in the larger overall manufacturing operation. Automated welding guns ordinarily perform this task.
Resistance spot welding is a joining process in which heat is generated by the resistance to the flow of electrical current through the sheet metal layers being joined. Typically, a pair of opposing welding electrodes is brought into contact with opposite sides of sheet metal layers at diametrically common spots (each spot sometimes being called the contact patch). A momentary electrical current is then sent through the sheet metal layers. Resistance to the flow of electrical current through the bulk sheet metal layers and their faying interface generates heat at the faying interface (i.e., the contacting interface), forming a molten weld pool which, upon stopping the current flow, solidifies into a weld nugget. The opposing welding electrodes also clamp the sheet metal layers under pressure to provide good electrical contact and to contain the molten weld pool at the intended weld site.
Automotive body panels made up of aluminum alloy sheet metal layers can present peculiar challenges for the spot welding step. For one, aluminum alloys are covered by oxide layers on their outer surfaces created both by processes experienced in mill operations (e.g., annealing, solution treatment, casting, etc.) as well as environmental exposure, which increases electrical resistance at the contact patch. Because of the high electrical resistance of the oxide layers and the relatively low thermal and electrical resistance of the underlying aluminum alloy, a high current level is typically required to form a weld pool at the aluminum alloy sheet metal layers' faying interface.
While helpful in forming a weld pool at the desired location, a high current level can create excessive heat at the contact patch which, in turn, may accelerate a metallurgical reaction between the aluminum alloy material and copper alloy material of the associated welding electrode. This reaction causes a contamination layer of copper-aluminum alloy to build-up or accumulate on the welding electrode. If left undisturbed, the contamination build-up can spall and form pits in the welding electrode, which ultimately will harm welding performance and complicate electrode dressing. These challenges, as well as others, oftentimes call for different considerations than spot welding workpieces made of steel.